1. Field of the Invention
The present invention relates to an in-plane switching (IPS) active matrix type liquid crystal display, and, more particularly, to a liquid crystal display which suppresses a reduction in contrast and color shifting when the viewing angle changes.
2. Description of the Related Art
An IPS liquid crystal display presents image display by forming an electric field, parallel to a liquid crystal substrate, between pixel electrodes and a common electrode, and has an advantage of providing a wider viewing angle over the TN mode type or the like, which forms an electric field perpendicular to the substrate. FIG. 1 shows the structure of a conventional IPS active matrix type liquid crystal display. As shown in FIG. a1, the liquid crystal display has a liquid crystal display (LCD) panel 10, a first polarizer 31 laid out on the top side of the LCD panel 10 and a second polarizer 32 laid out on the bottom side of the LCD panel 10. The LCD panel 10 comprises an active device substrate 11 on which scan lines 111, data lines 112, thin film transistors (TFTs) 113, pixel electrodes 114, a common electrode 115, a common electrode line 116, etc. are formed, an opposing substrate 12 on which a black matrix 121, color layers (color filters) 122, etc. are formed, and a liquid crystal layer 13 held sandwiched between the active device substrate 11 and the opposing substrate 12. As shown in FIG. 2A which is an exemplary cross-sectional view of the conventional IPS active matrix type liquid crystal display and FIG. 2B which shows the alignment direction of the liquid crystal layer 13 and the directions of adsorption axes of the first and second polarizers 31 and 32, the direction of the adsorption axis of the first polarizer 31 is set perpendicular to the alignment direction of the liquid crystal layer 13 and the direction of the adsorption axis of the second polarizer 32 is set parallel to the alignment direction of the liquid crystal layer 13.
According to the conventional IPS active matrix type liquid crystal display, the liquid crystal layer 13 has a birefringence if the viewing angle is changed even when no electric field is applied to the liquid crystal layer 13, so that the adsorption axes of the polarizers 31 and 32 do not appear to perpendicularly cross each other as observation is made obliquely. That is, the LCD panel 10 in a black display state causes a birefringence effect due to the apparent deviation between the alignment direction and the polarization plane that is caused by oblique observation of the liquid crystal layer 13. In case of oblique observation, the birefringence of the protection layer of the polarizer influences polarized light that passes the liquid crystal display. The polarizer comprises a polarization layer formed of a material having a polarization property and a protection layer which protects the polarization layer. It is known that triacetyl cellulose which is generally used to form the protection layer has an optical anisotropy during the fabrication process of the polarizer. The optical anisotropy causes birefringence with respect to light which passes the liquid crystal display at the time the viewing angle of the liquid crystal display is changed, thereby degrading the viewing angle characteristic. Such degradation increases the luminance in a dark state in case of conducting oblique observation, thus lowering the contrast. FIG. 3A shows the results of actually measuring the viewing angle characteristic for the contrast of the conventional liquid crystal display. As seen from the diagram, there is an area with a contrast of less than 5 as observation is made obliquely.
In case where oblique observation is made, the optical path becomes longer as will be discussed later with reference to FIG. 5B, so that the apparent retardation of the liquid crystal layer changes. When the viewing angle is changed, therefore, the wavelength of light which passes the liquid crystal display varies so that the colors on the screen look changed, thus causing so-called color shifting that depends on the direction of observation. FIG. 3B shows the results of measuring the viewing angle characteristic for the chromaticity of the conventional liquid crystal display with the conventional structure. As apparent from the diagram, the chromaticity varies significantly with a change in viewing angle. FIGS. 3A and 3B respectively correspond to FIGS. 6A and 6B.
Various schemes have been proposed to prevent a reduction in contrast and color shifting that depend on the viewing angle of such a conventional IPS type liquid crystal display. For example, Japanese Patent Laid-Open No. 133408/1999 has proposed a scheme of intervening a compensation layer having an optical anisotropy between a liquid crystal layer and a pair of polarizers which sandwich the liquid crystal layer. While this scheme effectively works on color shifting, however, the publication fails to mention that the scheme improves the contrast. Japanese Patent Laid-Open No. 2001-242462 has proposed a scheme of intervening first and second retardation plates between a liquid crystal layer and a pair of polarizers which sandwich the liquid crystal layer. Although the publication describes that the scheme effectively improves color shifting and the contrast, higher improvements are desirable.